The present invention relates to a thermosensitive recording material, and more particularly to a thermosensitive recording material with an improved thermal coloring sensitivity, which thermosensitive recording material comprises a support, an intermediate layer formed on the support, and a thermosensitive coloring layer formed on the intermediate layer.
Recently, the thermosensitive recording materials have been employed in a variety of fields, for instance, for use with recorders, terminal printers, facsimile apparatus, automatic ticket vending apparatus, and bar code readers.
In accordance with recent remarkable diversification of the application of the above-mentioned recording apparatus and the improvement of the performance, there is a great demand for thermosensitive recording sheets which can complement those improved apparatus. In particular, a thermosensitive recording sheet with an improved thermal coloring sensitivity is greatly demanded in accordance with the increased operation speed of facsimile apparatus. Thus, varieties of thermosensitive recording materials have been proposed to satisfy the above-mentioned requirements.
For example, as a method of increasing the thermal coloring sensitivity of the thermosensitive recording material, there is widely known a method of adding a thermofusible material to the conventional thermosensitive recording material in order to lower the coloring initiation temperature thereof, as disclosed in Japanese Laid-Open Patent Applications 49-34842, 52-106746 and 53-39139 However, the above-prepared thermosensitive recording material has the shortcomings that the background of such a thermosensitive recording sheet is also slightly colored at high temperatures, that dust is formed from the recording material in the course of thermosensitive recording by use of the thermal head which comes into contact with the surface of the thermosensitive recording material, and that the thermosensitive recording sheet sticks to the thermal head.
On the other hand, for the purpose of increasing the thermal coloring sensitivity of the thermosensitive recording material, there is also known a method of smoothing the surface of the thermosensitive coloring layer. By this method, the thermosensitive coloring layer of the thermosensitive recording material is brought into contact with the thermal head more closely, which causes the thermal energy provided by the thermal head to be efficiently absorbed in the thermosensitive coloring layer. This method of smoothing the surface of the thermosensitive coloring layer is usually carried out under application of pressure by use of a supercalender. Therefore, this method has the shortcomings that the background of the thermosensitive recording material is colored during the calendering process and the surface of the thermosensitive coloring layer becomes so glossy that the appearance of the recording material is impaired.
In addition, there have been proposed thermosensitive recording materials, in which an expanded layer with sufficient heat insulating properties is interposed between a support and a thermosensitive coloring layer for the purpose of effectively utilizing the thermal energy provided by a thermal head, for example, in Japanese Laid-Open Patent Applications 55-164192, 57-114658 and 59-171685. However, to improve the heat insulating properties of the above expanded layer, the voidage thereof must be increased. When the voidage of the expanded layer is increased, the surface smoothness of the expanded layer is degraded, with the result that the dots cannot be faithfully reproduced in images and recorded images are inferior in uniformity.
As an attempt to cover the above shortcoming that the surface smoothness of the expanded layer is decreased, there is proposed a method of overlaying on the expanded layer an undercoat layer comprising a filler and a binder agent, as disclosed in Japanese Laid-Open Patent Application 59-225987 This method, however, induces the complication of manufacturing processes of such a thermosensitive recording material, and accordingly the manufacturing cost is high.
Another attempt to compensate the above shortcoming is to subject the expanded layer to calendering for smoothing the surface thereof. This method has also the shortcomings that the manufacturing cost is high and that thermal conductivity of the expanded layer may be deteriorated.
Thus, a thermosensitive recording material having satisfactorily high coloring sensitivity, while maintaining high background whiteness and high heat insulating properties, has not been obtained yet.